The present invention relates to a method for fabricating a semiconductor device including an improved phosphorus getter treatment step.
In the fabrication of a semiconductor device, the phosphorus getter step which uses phosphorus oxychloride POCl.sub.3 is performed to eliminate the contamination and defects mainly in the substrate. The conventional phosphorus getter treatment will briefly be described with reference to the fabrication of a MOS dynamic RAM with a double-layered gate structure.
In the first conventional method shown in FIG. 1, a p.sup.+ -type layer 2 (channel stop region) is formed on a predetermined part of a p-type silicon substrate 1, and a field oxide film 3 is formed thereover by the selective oxidation method. In an island-shaped element forming region separated by the field oxide film 3, a first gate electrode 5 is formed on a first gate oxide film 4, and a second gate electrode 7 is formed on a second gate oxide film 6 and on a layer insulation film 8. After forming an n.sup.+ -type diffusion layer 9 as a digit line of the dynamic RAM in the substrate 1, an SiO.sub.2 film 10 is formed by the CVD process over the entire surface of the structure. The phosphorus getter treatment is performed at about 1,000.degree. C. while flowing POCl.sub.3 under this condition. Thereafter, contact holes 11.sub.1 and 11.sub.2 for connection with the first and second gate electrodes and a contact hole 12 for connection with the n.sup.+ -type diffusion layer are formed in the SiO.sub.2 film 10. However, with this conventional method, the position of the etching mask may be misaligned during the formation of the contact hole 12 to cause the field oxide film 3 to be etched. This exposes the p.sup.+ -type layer 2 through the contact hole 12, resulting in short circuiting. Furthermore, since the insulating film is the SiO.sub.2 film 10, the three-dimensional pattern of the underlying layer such as the gate electrode is directly transmitted to the surface of this insulating film. This causes the disconnection of an aluminum wiring layer which is formed on the surface of this insulating film.
Another conventional method as shown in FIG. 2 is proposed to solve these problems. According to this method, a field oxide film 23 having at its underside a p.sup.+ -type layer 22 is selectively formed in a p-type silicon substrate 21. A thermally oxidized film is grown on an island-shaped element region separated by the field oxide film 23. A polycrystalline silicon layer is deposited over the entire surface of the structure. After diffusing an n-type impurity in the polycrystalline silicon layer, it is patterned by photoetching to form a first gate electrode 25. Thereafter, the thermally oxidized film is etched using the electrode 25 as a mask to form a first gate oxide film 24. The thermal oxidation treatment is performed to grow a thick oxide film 28 as a layer insulating film. After removing the oxide film on the substrate 21, the thermal oxidation treatment is performed again and a thermally oxidized film as a second gate oxide film is grown on the exposed part of the substrate 21. After depositing a polycrystalline silicon layer over the entire surface and diffusing an n-type impurity therein, the polycrystalline silicon layer thus formed is patterned to form a second gate electrode 27. The thermally oxidized film is etched using this electrode 27 as a mask to form a second gate oxide film 26. An n.sup.+ -type diffusion layer 29 as a digit line is formed by diffusing an n-type impurity into the substrate 21. A contact hole for connection with the first gate electrode 25 is formed at part of the oxide film 28. A CVD-SiO.sub.2 film 30 and a boronphosphorus silicate glass film (BPSG film) 31 are sequentially formed over the entire surface. Then, contact holes 32.sub.1, 32.sub.2 and 32.sub.3 are formed at parts of the BPSG film 31 and the CVD-SiO.sub.2 film 30 which are on the contact hole described above, the second gate electrode 27 and the n.sup.+ -type diffusion layer 29. The phosphorus getter treatment is performed at about 1,000.degree. C. with POCl.sub.3. According to this conventional method, the BPSG film 31 is melted by the phosphorus getter treatment step and its surface is smoothed. Simultaneously, rediffusion of phosphorus in the n.sup.+ -type diffusion layer 29 is performed through the contact hole 32.sub.2. Due to this, the exposed p.sup.+ -type layer 22 is converted into an n.sup.+ -type diffusion layer and short circuiting may thus be prevented. Since, however, the gate electrodes 25 and 27 of polycrystalline silicon are exposed through the contact holes, POCl.sub.3 reacts with the exposed polycrystalline silicon, especially, with the impurities contained in the polycrystalline silicon. For this reason, the exposed parts of the gate electrodes are reduced in thickness and if a drastic reaction is involved holes will be formed. When the time for the phosphorus getter treatment is prolonged or the flow rate of POCl.sub.3 is increased, not only the gate electrodes but also the n.sup.+ -type diffusion layer 29 exposed through the contact hole 32.sub.3 are reduced in thickness and cracks are formed.